If Christians wish to partake in said activities then they should do so in a private area, whether it be a private Christian school, household, private Christian community groups etc. Having a secular state means the state takes no side in any religion, meaning that any 'public square' or grounds cannot be used for religion because it makes the impression of the impartial secular state favoring one religion over the rest, which is why teaching religion in public schools must be objective and give equal time to all major religions.